1. Field of the Invention
This invention relates to an improvement in a packaging multilayer plastic container comprising an interlayer of an ethylene/vinyl alcohol copolymer and an inner and an outer layer of a moisture-resistant thermoplastic resin. Particularly, it relates to an improvement of the aforesaid container for enhanced preservability under high humidity conditions and for increased adaptability to hot filling and heat-sterilization retorting after filling.
2. Description of the Prior Art
Ethylene/vinyl alcohol copolymer is one of those resins which have the best resistance to permeation to gases such as oxygen, and by utilizing this characteristic, has found extensive use in the fields of packaging containers such as bottles or cups or films. The ethylene/vinyl alcohol copolymer has the defect that it is sensitive to humidity, and at a high humidity of, for example, 100% RH, its coefficient of oxygen permeation becomes higher by about one order of magnitude. In order to remedy this defect, it has been the widespread practice to decrease the effect of humidity on the ethylene/vinyl alcohol copolymer by sandwiching a gas-barrier layer containing the ethylene/vinyl alcohol copolymer with an inner and an outer surface layer of a low water-absorbing resin such as polyethylene and polypropylene to provide a laminated structure.
With this laminated structure, direct contact of the ethylene/vinyl alcohol copolymer with water or moisture can be avoided, but moisture which permeates through the moisture-resistant resin layers exerts an unnegligible effect. For example, it is known that the gas-barrier property of the ethylene/vinyl alcohol copolymer is markedly reduced when the resulting container is heat-sterilized for increased preservability after an article is filled in it and the container is sealed.
Since the degraded gas barrier property cannot be restored even when the ethylene/vinyl alcohol copolymer is again dried, it is believed to be due to the impariment of the hydrogen bond between the polymer chains as a result of the copolymer undergoing the effects of both heat and humidity.
In addition, the production of containers of this type involves a problem of reutilizing scraps. The production of packaging containers from the aforesaid laminate necessarily generates scraps, for example burrs formed by pinch-off in blow molding of bottles or the like, and punching wastes in cup molding. Re-utilization of such scraps becomes necessary in order to save resources. The scraps, because of the aforesaid laminated structure, are composed of a blend of the ethylene/vinyl alcohol copolymer and the low water-absorbing resin. It is the usual practice to interpose this blend between the moisture-resistance resin layer and the ethylene/vinyl alcohol copolymer layer in the production of containers of the aforesaid multilayer structure. However, the use of the scraps leads to molded articles having very inferior properties, and it has been observed that the resulting articles are extremely reduced in, for example, gas-barrier property, mechanical properties such as strength, elongation and impact strength, flavor characteristics and outside appearance characteristics.
Recently, it was proposed, for example in U.S. Pat. No. 4,407,897, to provide a layer of a resin composition containing a desiccant between the moisture-resistant resin layer such as a polyolefin and the ethylene/vinyl alcohol copolymer layer. However, the desiccant generally has low moisture absorption at high humidities, and is not entirely satisfactory for the purpose mentioned.